Question about finding min. sum of product using K-maps?

2. The attempt at a solution
I understand you can simplify it with the Boolean theorems (e.g r't' + r = t' + r) , however how would you solve it using K-maps? I drew out a truth table, but it seems as if this SOP expression is simplified to the point where there is a missing variable for each term in the function to be able to plot the minterm within the k-map..